The UCLA Center of the Multicenter AIDS Cohort Study proposes to continue to document clinical, immunologic, physiologic, behavioral, virologic, genetic and psychosocial changes in HIV-infected and -uninfected men-who-have-sex-with-men (MSM). This includes proposed studies to document these changes in MSM recruited 1983-4 and 2001-2, as well as plans to recruit new untreated and recently treated HIV-infected MSM (primarily African- and Hispanic- Americans), to maintain the cohort. The UCLA MACS will provide leadership and participation in the working groups, MACS-wide and local studies and research publications, and collaborative multi-cohort studies, as it has since the inception of the MACS 30 years ago. The UCLA MACS has recruited a large team of young and established co-investigators from a wide range of disciplines. These investigators have developed, and are continuing to develop, innovative proposals to advance our understanding of the pathophysiology, immunology, genetics and biobehavioral characteristics of treated and untreated HIV infection. The breadth, youth, experience and innovation of the UCLA MACS investigators, combined with the experience and commitment of the long-term staff, and our leadership of quality control procedures for immunologic and virologic measures and specimen processing, will continue to play an invaluable role in the success of the MACS over the next five years.